1. Field of the Invention
The present invention relates to gauges, and in particular to thickness gauges that are capable of determining the thickness of a coating on a substrate, regardless of the composition of the substrate and coating.
2. Discussion of Related Art
Ultrasound provides an ideal physical mechanism to investigate the thickness of coatings on substrata with acoustically different properties. When a coating is applied to a substrate that has acoustic properties that are different from those of the coating, an acoustic coating/substrate interface is created. At such an interface, an ultrasonic vibration will be partially reflected.
For example, an ultrasonic vibration, also known as an impulse, can be transmitted into a coating using a resonant piezo element transducer. The same transducer can also be setup to "listen" for echoes created when the impulse reflects from the interface of the coating and substrate back to the transducer. The output of the transducer can be recorded for a known period after the impulse has been transmitted. This period is defined as an echo window. The echo window is defined to overlap with the time of expected echoes of interest.
By analyzing the echo recorded during the echo window, it is possible to determine the location of the interface between the coating and the substrate. The thickness of the coating can be determined if the velocity of sound within the coating material and the time of the interface echo are known. In other words, the thickness of the coating can be determined by multiplying the velocity of the vibration through the coating material times the time for the vibration to enter the coating, reflect off the interface, and exit the coating, and dividing that product by two. EQU Thickness=(Velocity.times.Time)/2
The resolution of the derived thickness is limited by the temporal resolution of the sampled echo. Improvements in the resolution of the sampled echo will directly improve the resolution of the derived thickness.
Ultrasonic coating thickness gauges used to measure coatings on nonferrous, nonconductive substrata have existed for some time. The gauges fall into three broad categories: Real Time Echo Analyzers, Real Time Echo Capture/Digital Analyzers, and Analog/Digital Hybrid Flaw Analyzers. A brief description of each of the types of gauges follows:
1. REAL TIME ECHO ANALYZERS
Real time echo analysis refers to gauges that produce ultrasonic impulses and attempt to analyze the resulting echoes in real-time. Ultrasonic thickness gauges designed to measure wall thickness typically use a gated threshold detector to enable a measuring circuit during the period of transducer/material echo and material/air echo (back echo). The measuring circuit typically is required to resolve to a resolution of 10 nsec or better. For example, to resolve to a resolution of 0.001 inch in STEEL 410 would require a resolution of: EQU 1 mil/291 mil/s=1/291*2(round trip)=6.87 nsec.
Typically, wall thickness gauges of this type derive an average based on a large number of period (transducer/material echo to material/air echo) measurements. Some gauges retrigger the pulser-upon the detection of back echoes. A predetermined number of retrigger cycles are allowed to occur while a timer determines the length of time for the predetermined number of cycles to occur. By increasing the number of retrigger cycles it is possible to achieve very good resolution.
Methods to precisely measure the delay time between the transducer/coating echo and coating/substrate echo exist. U.S. Pat. No. 4,685,075 "APPARATUS FOR MEASURING PROPAGATION TIME OF ULTRASONIC WAVES" and U.S. Pat. No. 4,838,086 "METHOD FOR MEASURING THE WALL THICKNESS OF A WORKPIECE BY ULTRA-SOUND" illustrate several techniques for resolving the delay between multiple echoes. This style of gauge requires "crisp echoes" from both surfaces. This type of gauge provides inconsistent results when measuring coatings on substrata that have acoustic properties that are similar to those of the coatings.
To help overcome this problem most gauges of this type utilize a variable threshold detector that is ramped down during the echo period, For Example, while the ultrasonic vibration is transmitted into the material, the threshold is maintained at a level that will not allow the detector to trigger. As the ultrasonic vibration propagates through the coating the threshold level is decreased. In this way the gauge will be less susceptible to transducer ring from the transducer/material echo and can compensate to coating attenuation of the vibration. However, this problem has not been adequately addressed, particularly when thin coatings are being measured. Most ultrasonic gauges of this type, although well suited for particular applications, lack the flexibility to be adapted for multiple applications.
2. REAL TIME ECHO CAPTURE/DIGITAL ANALYSIS.
To overcome some of the drawbacks associated with real time echo analysis, real time echo capture gauges are used to digitize the echo waveform. Typically, this type of instrument incorporates a very high speed A/D converter to digitize the echo waveform in real time. For example, to resolve to a resolution of 0.001 inch in STEEL 410 would require a resolution of 6.87 nsec. The A/D converter required must be capable of sampling the waveform at this resolution.
Typically, gauges that use these techniques are confined to lab use where high speed sampling scopes and digital computers can be employed to digitize and analyze the resulting echo waveforms.
3. ANALOG/DIGITAL HYBRID FLAW ANALYZERS
Flaw analyzers typically provide mechanisms to generate, amplify, and display the echo waveform of an ultrasonic inspection. Such instruments are very useful in the investigation of subsurface flaws in metal structures. Flaws such as subsurface cracks or voids can be imaged very accurately. For example, welding flaws and corrosion on the inside diameter of a pipe used to carry toxic materials can be imaged with analog/digital hybrid flaw analyzers.
Such instruments, although very useful, require the operator to understand/interpret the results. The operation of such equipment for coating thickness gauging requires the operator to distinguish from the echo window the interface echo of interest. Once the echo of interest has been found, a scale can be used to directly measure distance based on the velocity of sound in the material.